All soils and water contain various metals that vary in concentrations. Mg is a macronutrient and is essential for enzymatic activities relating to photosynthesis. A substantial amount of Mg is found in plant tissue, hence the term macronutrient (Manahan, 2010). Metals, such as Fe, Mn, Cu, Zn, and Ni are essential micronutrients for many living organisms (Cempel and Nikel, 2006; Göhre and Paszkowski, 2006; Manousaki and Kalogerakis, 2011; Kalogeropoulos et al., 2012), however at high concentrations the metals may lead to toxic effects (Duffus, 2002; Hodson, 2004). Metals that are considered to be non-essential includes Cd, Pb, As, Hg and Cr (Mertz, 1981; Kärenlampi et al., 2000; Suzuki et al., 2001; Cobbett, 2003; Peng et al., 2009; Sánchez-Chardi et al., 2009; Dabonne et al., 2010). Such metals are not required for either physiological and/or biochemical functioning of a living organism (Ali et al., 2013).
Problems with Toxic Metals
The problem associated with metals is that they are not biodegradable and therefore tend to persist in the environment and become mobile and bioavailable (Duruibe et al., 2007). Metals tend to accumulate within living tissues of different organisms. This is known as bioaccumulation. Bioaccumulation tends to increase in concentration from low trophic organisms to higher trophic organisms (see figure 1). This is termed as biomagnification (Ali et al., 2013). There have been many reports stating that toxic metal concentrations in soils cause long term hazardous effects on soil biological processes, which result in negative changes to soil ecosystems (Spier et al., 1999; Lorenz et al., 2006; Malley et al., 2006). Authors, such as (Khan et al., 2007) indicate that toxic metal concentrations have inhibitory effects on soil enzymes (soil activity) that causing structural changes within microbial communities. Thus soil microbes tend to decrease in numbers as a result of toxicological effects due to the presence of such metals (Khan et al., 2010). All the above suggest that toxic metal contamination within natural environments deserves special attention, especially since the effect is observed at relatively low concentrations (Arora et al., 2008; Kara, 2005; Memon and Schroder, 2009).
The most toxic forms of metals are when they are in their most stable oxidative state, as they are able to react with biomolecules and form compounds that are biotoxic (Duruibe et al., 2007). This is termed oxidative stress (Mudipalli, 2008). Oxidative stress occurs when free radicals are formed when metal molecules react with oxygen. Free radicles are known to be atoms that contain only one unpaired electron. Free radicals are capable to exist independently.
Sources of toxic metal concentrations
Natural and anthropogenic sources of pollution contribute to toxic metal contamination, but the most significant causes are due to anthropogenic activities. Metal accumulation leading to toxic effects occurs when there is an...